A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a polymer electrolyte membrane (PEM) that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce protons that pass through the PEM. The electrons produced by this reaction travel through a circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the protons to form water.
A typical fuel cell has a terminal voltage of up to approximately one DC volt. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack.
A fuel cell system may include a fuel processor that converts a hydrocarbon (e.g. methane) into a fuel flow for the fuel cell stack. Exemplary fuel processor systems are described in U.S. Pat. Nos. 6,207,122, 6,190,623 and 6,132,689, which are hereby incorporated by reference.
The two reactions which are generally used to convert a hydrocarbon fuel into hydrogen are shown in equation (1) and (2).½O2+CH4→2H2+CO  (1)H2O+CH4→3H2+CO  (2)
The reaction shown in equation (2) is generally referred to as stem reforming. Both reactions may be conducted at a temperature from about 600° C. to 1,100° C. in the presence of a catalyst such as platinum (Catalytic Reformer). CO produced by these reactions is generally present in amounts greater than 10,000 ppm. Because of the high temperature at which the fuel processor is operated, this CO generally does not affect the catalyst in the fuel processor. However, if this reformate is passed to the prior arts fuel cell system operating at a lower temperature (e.g., less than 100° C.), the CO may poison the catalyst in the fuel cell by binding itself to catalyst sites, inhibiting hydrogen in the cell from reacting. In such systems it is typically necessary to reduce the CO levels to less than 100 ppm to avoid damaging the fuel cell catalyst. For this reason the fuel processor may employ additional reactions and processes to reduce the CO that is produced. For example, two additional reactions that may be used to accomplish this objective are shown in equations (3) and (4). The reaction shown in equation (3) is generally referred to as a Shift Reaction, and the reaction shown in equation (4) is generally referred to as a Preferential Oxidation (PROX).CO+H2O→H2+CO2  (3)CO+½O2→CO2  (4)
Various catalysts and operating conditions are known for accomplishing the shift reaction. For example, the reaction may be conducted at a temperature from about 300–600° C. in the presence of supported platinum. Other catalysts and operating conditions are also known. Such systems operating in this temperature range are typically referred to as high temperature shift (HTS) systems. The shift reaction may also be conducted at lower temperatures such as 100–300° C. in the presence of other catalysts such as copper supported on transition metal oxides. Such systems operating in this temperature range are typically referred to as low temperature shift (LTS) systems. Other catalysts and operating conditions are also known. In a practical sense, typically the shift reaction may be used to lower CO levels to about 1,000–10,000 ppm, although in an equilibrium reaction it may be theoretically possible to drive CO levels even lower.
The PROX reaction may also be used to further reduce CO. The PROX reaction is generally conducted at temperatures lower than the shift reaction, such as at 100–200° C. The PROX reaction can also be conducted in the presence of an oxidation catalyst such as platinum or alternatively platinum supported on alumina (e.g., in the form of Al—O—Pt). The preparation of the latter catalyst comprises for example the steps of (a) washcoating a honeycomb with alumina (Al2O3) in the form of slurry, drying the washcoating and then impregnating the washcoating with platinum to produce a catalyst platinum supported on alumina and (b) calcinating the catalyst platinum supported on alumina obtained from step (a) at 300˜700° C. This catalyst shows much difference in the ability of reducing the concentration of carbon monoxide in the PROX reaction depending on the amount of platinum used and its preparation. Nevertheless, these catalysts could not stably reduced about 1˜6% of carbon monoxide under H2 to less than 50 ppm. Thus, in the PROX reaction, there is a continuing need for a PROX reaction catalyst in which a PROX reaction catalyst can stably reduce the concentration of carbon monoxide to produce a treated fuel gas stream comprising less than about 50 ppm carbon monoxide.